In a continuous hot-dip plating plant, in which a steel plate is plated by being continuously immersed in a hot-dip metal plating bath, it is necessary that the steel plate is immersed in the hot-dip metal plating bath while maintaining the steel plate at a temperature suitable for plating and keeping the surface thereof in a non-oxidized state. For this reason, equipment having a rectangular cross section, which is referred to as snout, is formed between a continuous annealing furnace outlet and the hot-dip metal plating bath.
Since a bottom end of the snout reaches a bath surface of the hot-dip metal plating bath, metal vapor of molten metal generated on a molten metal surface is cooled on a surface of a wall of the snout, and is coagulated and deposited. When this drops, due to the self-weight, vibration, or the like, on the steel plate and attaches to the steel plate, or on the bath surface of the hot-dip metal plating bath and then attaches to the steel plate, it may cause quality defect which is referred to as unplating that a part of the steel plate is not plated. Further, the metal vapor of the molten metal is condensed and formed into a particle shape (having a size of 1 μm or less in many cases), and after having been deposited on the surface of the wall of the snout as metal fumes, is dropped on and attached to the steel plate or is directly attached to the steel plate, which causes a similar quality defect. Those metal vapor and metal fumes further gather together and form metal dust (having a size of 1 μm or more in many cases), which causes a more serious quality defect.
Accordingly, as shown in Patent Document 1, there is suggested technology that an electric heater is installed around a snout to heat the snout from an outside. When this method is adopted, the temperature of the inner wall of the snout rises, and hence, the amount of coagulation and deposition of metal fumes is reduced. However, the amount of coagulation and deposition does not become zero, and metal vapor evaporated from the molten metal surface is continuously coagulated and deposited on the inner wall of the snout, which will eventually drop and cause unplating.
Further, since the heating is performed from an outside using the electric heater, the temperature of the outer side of the snout becomes higher than the temperature of the inner side of the snout, and thus, thermal deformation of the snout easily occurs. When the shell of the snout cracks due to such thermal deformation and the air enters, this also causes quality defect.
In addition, Patent Document 2 suggests, as shown in FIG. 1 and FIGS. 2A to 2C, technology in which an exhaust port 2 is provided to each of the both left and right sides of a lower part of a snout 1 to exhaust atmospheric gas containing metal vapor evaporated from a molten metal surface, the metal vapor is condensed and separated using a separator 3, and then only the atmospheric gas is sent back to the inside of the snout through an air inlet port 4 provided to a position on each of the both left and right sides of an upper part. However, in this arrangement, a short path for the atmospheric gas is formed between the air inlet port 4 and the exhaust port 2, as shown in FIG. 2A. Accordingly, it is not possible to maintain the stream that can shut off the metal vapor in the central part of the snout 1. Therefore, some of the metal vapor slips through the central part of the snout to the upper part as shown by an arrow 5, and remains inside the snout 1. Further, some of the metal vapor that has slipped through to the upper part moves toward the continuous annealing furnace, and coagulates and deposits on the inner wall thereof, which causes unplating in the same manner.